(a) Field of The Invention
The present invention relates to a large aperture ratio photographic lens system and, more particularly, to a large aperture ratio photographic lens system which enables to obtain high quality of image for an object at the infinite distance up to an object at an extremely short distance where the photographing magnification is 1/2 or more.
(b) Description of The Prior Art
Gauss type and modified Gauss type lens systems have high degree of freedom in correction of aberrations and are advantageous as large aperture ratio photographic lens systems. Besides, as the above-mentioned types of lens systems are symmetrical in the lens composition, they enable to reduce the variation of aberrations related to the variation of distance to the object, i.e., variation of magnification. However, when the lens system is arranged to have a large aperture ratio and extremely short distance photographing is carried out by that lens system, the quality of image becomes conspicuously unfavourable, and variation of aberrations becomes large at the magnification about 1/5 and higher.
As methods for preventing the variation of aberrations to be caused when the photographing magnification is varied, it is known to widen the airspace, where the stop is arranged, when the photographing magnification is made higher or to widen one of airspaces in rear of the stop at the same time as widening the above-mentioned airspace where the stop is arranged. Lens systems disclosed in Japanese published unexamined patent applications Nos. 46221/82, 8513/82, 107208/81, 192916/82 and 28038/80 are known as the lens system in which the above-mentioned methods are adopted.
However, to reduce the variation of spherical aberration to be caused when the airspace where the stop is arranged is widened as in cases of the above-mentioned methods, it is necessary to decide the refractive power of the lens group arranged in front of the stop so that paraxial rays, which come out from the lens group arranged in front of the stop, will not diverge so much even when the distance to the object becomes short.
On the other hand, when a Gauss type or modified Gauss type lens system is arranged to have a large aperture ratio and, at the same time, to have a satisfactorily long back focal length, there is a general tendency that the refractive power of the front lens group arranged in front of the stop becomes considerably weaker than the refractive power of the rear lens group arranged in rear of the stop. Besides, when arranging a lens system to have a large aperture ratio, it is difficult to correct aberrations unless thicknesses of respective lenses constituting the lens system, especially, thicknesses of respective lenses constituting the rear lens group are made large and, consequently, the back focal length necessarily becomes short. This means that, to obtain a satisfactorily long back focal length in the state to photograph an object at the infinite distance, the refractive power of the front lens group arranged in front of the stop should be made still weaker. Therefore, there is no known lens system put to practical use which has a large aperture ratio similar to ordinary standard type lens systems and, at the same time, enables to photograph an object at an extremely short distance.